1. Field of the Invention
The present invention relates to a method of dielectrically heating an adhesive. More specifically, the present invention relates to a method of monitoring the state of cure in a dielectrically heatable adhesive.
2. Description of the Related Art
It is known to dielectrically heat an adhesive between two electrodes. A radio frequency (RF) electric field is created by an RF generator. The RF field causes polar and ionic molecules within the adhesive to oscillate and create heat within the adhesive. The method of dielectrically heating an adhesive is described in U.S. Pat. No. 5,277,737, and is incorporated herein by reference. Heretofore, the amount of dielectric heating has either been analytically calculated based on known formulas and properties of the materials being heated, or determined by experimentation. Process variables, including the exact composition of the dielectrically heatable adhesive, thickness of the adhesive layer, the dielectric constant of the members being joined, and environmental factors such as ambient temperature and humidity all influence the rate of dielectric heating. These process variables have not been accounted for in prior devices.
The amount of dielectric heating is directly related to the state of cure of the adhesive. This state of cure determines the amount of dielectric heating required to achieve a desired bond strength. It has previously been known to measure the plate current between the two electrodes to estimate the state of cure of the adhesive. The plate current measurement is an estimation of the average state of cure of the entire bond line. It does not measure the state of cure of a particular area. Sections that have less adhesive, or that are more susceptible to dielectric heating are averaged into the plate current measurement. These areas may experience over or under heating.
It is also known to embed a fiber optic temperature sensor within the adhesive to continuously monitor its temperature. Contact temperature measurements such as the photo optic method described require access to the adhesive through an apature in the part and very expensive and delicate instruments. This temperature measurement process is generally only suitable for experimental or developmental purposes. The temperature sensor often is damaged or trapped in the adhesive.
It is desirable to measure the adhesive temperature without contacting the adhesive or the members to be bonded. It is further desirable to measure the temperature at any point along the mating surfaces between the members. These and other advantages, features and objects of the present invention will be more fully described in the following specifications with reference to the attached drawing.